1. Field of the Invention
The invention relates to optical systems, and more particularly to an arrangement of optical elements for performing optical signal processing.
2. Description of Prior Art
The use of optical elements for simple, coherent optical signal processing is well known in the art. Processing functions such as matrix multiplication, Fourier transform, and convolutions can be performed using coherent optical processing. Such systems have been constructed from bulk three dimensional elements such as lenses, bulk modulators, and two dimensional detector arrays. Another important application is the spectral analysis of RF signals.
An optical RF spectrum analyzer described in the prior art employs the interaction between a coherent optical wave and an acoustic wave driven by an input electrical signal to determine the power spectral density of the input. Such an analyzer may be implemented in an integrated optics version, and is described in the article Integrated Optic Spectrum Analyzer, M. K. Barnowski, B. Chen, T. R. Joseph, J. Y. Lee, and O. G. Rama, IEEE Trans. on Circuits and Systems, Vol. CAS-26, No. 12, Dec. 1979. The integrated optics version consists of an injection laser diode, a thin-film optical waveguide, waveguide lens, a surface acoustic wave transducer, and a linear detector array.
The unit operates by mixing an incoming radar signal with a local oscillator such that the intermediate frequency is within the pass band of the transducer. After amplification, the signal is applied to the SAW transducer. The resulting surface acoustic waves traversing the optical waveguide generate a periodic modulation of the refractive index of the waveguide mode. If the culminated optical beam intersects the acoustic beam at the Bragg angle, a portion of the beam will be defracted or deflected at an angle closely proportional to the acoustic frequency with intensity proportional to the power level of the input signal. The Bragg detector light is then focused on an array of focal plane detectors where each detector output becomes one frequency channel of the spectrum analyzer. Such systems are limited to obtaining the intensity of the Fourier transform which is useful for determining the intensity of the incoming signal. However, the Fourier transformer alone and the knowledge of the intensity is insufficient to determine the amplitude of the individual frequency components.